Medical imaging of metabolic and biochemical activity within a patient is known as functional imaging. Functional imaging techniques include, for example, nuclear imaging such as Positron Emission Tomography (PET), Single Photon Computed Tomography (SPECT), functional magnetic resonance imaging (fMRI), and functional computed tomography (fCT). An overview of SPECT, PET systems, their combination with computer tomography (CT) systems as well as iterative image reconstruction for emission tomography is given in chapter 7, chapter 11, and chapter 21 of M. Wernick and J. Aarsvold, “Emission tomography: the fundamentals of PET and SPECT,” Elsevier Academic Press, 2004, the contents of which are herein incorporated by reference.
In general, SPECT imaging is performed by using a gamma camera to acquire multiple two-dimensional (2D) projections and then using a computer to perform tomographic image reconstruction to obtain a three-dimensional (3D) image. For example, a gamma photon-emitting radioisotope may be introduced into a patient's body, and any of various techniques can be used to bind the radioisotope to a location of interest in the body. The patient lies on a bed, and one or more gamma cameras are attached to the gantry which rotates, causing the gamma camera(s) to rotate around the patient. Detectors of the gamma camera(s) acquire projection data at each orientation by detecting gamma photons emitted by the radioisotope.
SPECT imaging devices are generally equipped with a collimator such as a parallel-hole, fan-beam, or cone-beam, or coded aperture (CA) collimator. In the case of parallel hole collimation the point response function focuses on a small area, so that each detector pixel mainly receives photons from the incoming gamma photon beam in the normal (i.e., perpendicular) direction. In the case of CA collimation, a thin shield with numerous holes (apertures) drilled in a certain pattern is placed parallel to the surface of a detector. With CA collimation, a detector pixel receives flux from various locations, so the projections from different regions of the source overlap, introducing complexities to the task of tomographic image reconstruction.